If a new mobile communication scheme such as a Long-Term Evolution (LTE) scheme, a Long-Term Evolution-Advanced (LTE-A) scheme, or the like, is introduced in a mobile communication scheme, the mobile communication scheme provides a limited service coverage area during an interval before establishing a whole network. If a network in which the new mobile communication scheme is introduced supports a Voice over Internet Protocol (VoIP) service, a voice call which a User Equipment (UE) receives is terminated due to a limitation of service coverage area when the UE moves out of the service coverage area.
In order to address this problem, a 3rd Generation Partnership Project (3GPP) standard has defined a Voice Call Continuity (VCC) scheme in which a UE may maintain a voice call through a voice handover between a new Packet Switching (PS) network and an old Circuit Switching (CS) network which has a relative large service coverage area even though the UE moves out of the service coverage area.
The VCC scheme defined in the 3GPP standard is based on a dual radio scheme. That is, the VCC scheme defined in the 3GPP standard assumes a UE which may simultaneously access and communicate with two different access networks, e.g., a PS access network and a CS access network, and defines a process provide continuity for a voice call if the UE hands over to the CS access network while receiving the voice call in the PS access network, or the UE hands over to the PS access network while receiving the voice call in the CS access network.
First, upon determining that there is a need for a voice handover from an access network, which a UE supporting the dual radio scheme currently accesses, to a new access network, the UE supporting the dual radio scheme transmits a message which requests a domain transfer to an Internet Protocol (IP) Multimedia Subsystem (IMS) server through the new access network.
If the new access network is the CS network, the UE supporting the dual radio scheme uses a CS setup message as the message which requests the domain transfer. If the new access network is the PS network, the UE supporting the dual radio scheme uses a Session Initiation Protocol (SIP) invite request message as the message which requests the domain transfer.
The IP Multimedia Subsystem (IMS) server connects a new bearer to the UE supporting the dual radio scheme through the new access network, and performs a domain transfer operation which switches a data path from an old bearer to a new bearer. Upon completing the new bearer connection, the IMS server notifies the UE that the domain transfer operation has completed by transmitting a SIP bye request message or a CS connection release message to the UE.
FIGS. 1A and 1B schematically illustrate a method for switching a voice call for a VCC UE in a case in which another party UE is an IMS UE in a conventional mobile communication system according to the related art.
Referring to FIGS. 1A and 1B, the IMS UE denotes a UE supporting an IMS scheme, and the VCC UE denotes a UE supporting the VCC scheme. The conventional mobile communication system includes a Domain Transfer Function (DTF) entity, a Media Gateway Control Function (MGCF) entity, a Serving-Call State Control Function (S-CSCF) entity, a Media Gate Way (MGW), a Visited Mobile Switching Center (VMSC), a VCC UE #A and an IMS UE #B.
As described in FIG. 1A, if the VCC UE #A detects that there is a need for switching a voice call during a voice call service operation with the other party UE, i.e., the IMS UE #B (111), the VCC UE #A transmits a message which requests domain transfer to the DTF entity. After receiving the message which requests the domain transfer from the VCC UE #A, the DTF entity transmits a message which indicates that the VCC UE #A requested the domain transfer to an IMS server (not shown in FIGS. 1A to 1B) (113). After receiving the message which indicates that the VCC UE #A requested the domain transfer from the IMS server, the IMS server performs a control operation in which the IMS server connects a new bearer to the VCC UE #A (115).
As described in FIG. 1B, after the new bearer between the IMS UE #B and the VCC UE #A is connected, a connection between the IMS UE #B and the VCC UE #A is performed through the new bearer between the IMS UE #B and the VCC UE #A (117), and a connection between the IMS UE #B and the VCC UE #A through an old bearer is terminated (119).
FIGS. 2A and 2B schematically illustrate a method for switching a voice call for a VCC UE in a case in which another party UE is a CS UE in a mobile communication system according to the related art.
Referring to FIGS. 2A and 2B, the CS UE denotes a UE supporting a CS scheme. The mobile communication system includes a DTF entity, an MGCF #1 entity, an S-CSCF entity, an MGCF #2 entity, an MGW #1, an MGW #2, a VMSC, a VCC UE #A and a CS UE #B.
As described in FIG. 2A, if the VCC UE #A detects that there is a need for switching a voice call during a voice call service operation with the other party UE, i.e., the CS UE #B (211), the VCC UE #A transmits a message which requests domain transfer to the DTF entity. After receiving the message which requests the domain transfer from the VCC UE #A, the DTF entity transmits a message which indicates that the VCC UE #A requested the domain transfer to an IMS server (not shown in FIGS. 2A to 2B) (213). After receiving the message which indicates that the VCC UE #A has requested the domain transfer from the IMS server, the IMS server performs a control operation in which the IMS server connects a new bearer to the VCC UE #A (215).
As described in FIG. 2B, after the new bearer between the CS UE #B and the VCC UE #A is connected, a connection between the CS UE #B and the VCC UE #A is performed through the new bearer between the CS UE #B and the VCC UE #A (217), and a connection between the CS UE #B and the VCC UE #A through an old bearer is terminated (219).
As described above, FIGS. 1A and 1B illustrate a User-plane (U-plane) path (data transmission path) and a switching point through a CS access network and a PS access network if the other party UE is an IMS UE, and FIGS. 2A to 2B illustrate a U-plane path (data transmission path) and a switching point through a CS access network and a PS access network if the other party UE is a CS UE.
If a UE supporting a DRVCC scheme performs a domain transfer operation, an anchor point which switches a path in a network may be changed according to whether the other party UE is a CS UE or a PS UE. That is, if the other party UE is the CS UE, an MGW for the UE supporting the DRVCC scheme becomes the anchor point. On the other hand, if the other party UE is the IMS UE, an MGW for the other party UE or an end point of an Internet Protocol Connectivity Access Network (IP-CAN) becomes the anchor point. The end point of the IP-CAN may be a Packet Gate Way (PGW) or a Gateway General Packet Radio Service (GPRS) Support Node (GGSN) according to the kind of access network involved.
As described above, in the DRVCC scheme, a domain transfer operation in which a voice data transmitting/receiving path is switched from an old bearer to a new bearer on a network, and an operation in which a voice data transmitting/receiving path is switched between two access networks within a UE are independently performed. That is, the UE may not know when a domain transfer occurs after a new bearer is connected.
In a currently proposed standard, it is not defined when a UE switches a path for transmitting/receiving voice data. So, the UE merely infers that the path for transmitting/receiving the voice data is switched from the old access network to the new access network upon receiving a SIP bye request message or a CS connection release message.
However, if a switching point in the access networks is not identical to a switching point in the UE, data loss necessarily occurs. That is, e.g., even though the switching point in the access networks is identical to the switching point in the UE, data loss occurs due to basic transmission delay between the UE and the access networks. Further, if the switching point in the access networks is not identical to the switching point in the UE, data loss quantity increases. For example, if a Node B transmits voice data through a new path and the UE receives the voice data through an old path, data loss occurs.
This means that, if voice data is lost while a voice service is provided in the mobile communication system, a voice call is dropped, so voice quality of the voice service deteriorates.
In the related art, an interval in which a domain transfer operation is performed in the mobile communication system is expected to be about 800˜1000 msecs according to the kind of and the particular situation of the access network, and an interval in which data loss may occur is expected to be tens to hundreds of msecs according to a gap between data path switching time for an access network and data path switching time for a UE. The data loss interval of tens to hundreds of msecs is time during which the user of the UE may recognize and experience discomfort.
Therefore, a need exists for a method and an apparatus for effectively switching a voice call in a mobile communication system that supports a DRVCC scheme.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.